1st, you'll want to upgrade your speakers to something that can handle, preferably, 50 watts RMS. that means 5.25" front, 6x9 (or 6.5" with adapters) in back (I believe those are the sizes). See how that sounds.
2nd: If you don't like the new sound, you'll have to either a) butcher your current harness or b) get a bose harness female to bose harness male to cut into. then, run the outputs (from radio) into a 4 or 4.1 channel amp and the the amp output to your speakers , keeping all of your wiring at the harness level. you might have room in the front to put the amp, behind & underneat the console area if your bonnie is like mine. I haven't really been inside the 2000'* yet.
You can also run power to your amp from your harness, but that presents a new set of problems and is a new thread that can be started if there is interest.
IMPORTANT: when you buy an amp, make sure it has bass controls, or bass gain, for your 4 channels. Then you'll get bass, and it won't be all treble, a very common problem when people put aftermarket amps in. Keep in mind, also that a few companies make a 4.1 channel amp....4 channels for mid range and highs, and 1 channel dedicated for a sub. always nice for expansion later in the game.
I would recommend this amp, or something similar: http://www.crutchfield.com/*-1RmZvoG...fo&i=158XM4060
'95 SSEi, few electronic mods (and a stupid coupler)
HU (Sony MDX-CA680, European Ver; Sony 4x3-ways; 91, 93dB)
Yup, that'* ISU